When you start service DB 80502 on Ruhr sieg nord at Lethmate, the signal stays red. When contacting the signaler, you may proceed at restricted speed but the signal stays red. Restarted the scenario couple times (even restarted the game) but no change.
You first have to reverse to drop off some wagons in the Letmathe 145 siding (check the in-game map if you don't know the location) and wait for a passenger service to pass you, then you may proceed.
On German routes, setting aside PzB issues, signaler clearance should add two diagonal white lights to the red (shunting clearance, proceed restricted). If that's not appearing, then I suspect the game isn't really giving you signaler permission- a bug.
I just finished the service, you we're right. I didn't expect to reverse the train to that siding, i'll be checking the map for sure next time. Thanks for you're help. Case closed